Problem with vinyl coated steel cables - cable slipped!

`While using my zip line the other day, tension was lost on the line. I checked the connections and found that the vinyl coated steel cable had slipped where it was clamped. The vinyl part ripped and the cable slid out. This is a serious safety issue. I recommend against using vinyl coated cable for critical applications. To get a more secure attachment, the vinyl must be stripped from the cable.
This is the cable I was using:

Everbilt

Model # 810632

Internet # 204394877

Store SKU # 454347

1/8 in. x 30 ft. Vinyl-Coated Wire Rope Kit

Moved Zip Line

I moved the zip line to the other side of my backyard. This time I decided to make it much simpler. I used the concrete/rebar anchors like before but made some modifications. For the start, I used one 4x6 post placed 1' deep and balanced by two chains. For the end, I used two posts with a cross member on top attached to one chain/anchor. This way the kids don't run into the post at the end. I used 1/2" eye bolts on either end to attach the cable. I used some shackles and additional cable  and hardware to connect everything. Here is the maiden run video:

I still have to install rungs to climb up the start pole and a cable stop or bumper at the end but I am pretty happy with this. It took much less effort than my last design.

Zip-Line Build Pics: posts, chain, seat, bumper, steps

Here is the starting pressure treated lumber post for the zip-line. I used two ground anchors which join together and attach to the wire. The first anchor position was not in line with the zip-line and it caused too much lateral force on the pulley resulting in a broken pulley. I used the second ground anchor to properly align the system. The horizontal section of chain 2/3rds up was used to increase line tension and balance bending forces on the post. Note that the post is angled backwards about 10 degrees from vertical. This also reduces bending forces. Two pulleys are used to reduce the force on each pulley. I used 1" diameter oak dowels as steps on the post so that the kids could climb up. It is difficult at first to balance but they quickly got the hang of it.

Here is the disk swing seat tied to the trolley of the zip line.

Here is the end post of the zip line showing the pulley on top and the chain to the ground anchor.

I needed a pad to wrap around the end post so that the kids don't get hurt. I looked at pads for basketball poles but they were very expensive. I found a camping pad and wrapped it around the pole with the included straps. It is doubled over itself to increase thickness. It works great. I ended adding a few more straps to secure it better than shown in the pictures.

You can get a camping pad like this at Amazon: 

Zip-line Post placed in crushed stone

I used a 4x6" pressure treated wood post 12' length. The hole was dug approx 4-1/2' deep, 2' across and 2' wide. A small amount of crushed stone was placed at the base and impacted. Then the post was set. It was angled away from the opposite post 5-10 degrees. Then the crushed stone was placed and impacted around the post using a 2x4 and bricks. The stone was finally covered with dirt and grass.

Concrete Anchor for Zip-line

This is my design for a concrete anchor to hold the tension of the zip line. I don't know how much weight this design can hold. It probably depends on ground conditions, depth of hole, geometry of hole, amount and type of cement, amount and type of rebar, and chain size and orientation.
I dug a hole about 2 feet deep, 3 feet across, and 2 feet long. I tried to make sure the base of the hole was wider than the top of the hole. Two rebar rods were impacted in the base of the hole angling away from the chain direction as shown. A third rebar rod was threaded through the last link on the heavy chain and placed behind the upright rebar rods. This arrangement is able to hold tension without any cement. Next, two 80 pound bags of concrete mix are prepared and poured into the hole. Dirt is packed on top of the anchor once the concrete sets.

Parts List for Zip Line installation with Wood Posts

I researched options for posts for the zip line in the net. At first I thought I would use pressure treated wood and put them in concrete. It seems that the wood will rot and the concrete may crack. Most people suggested placing the posts in the ground filling the hole with crushed stone. This allows the wood to stay dry and not rot. Since the posts are not subjected to high bending forces in my design, I thought this would work well. The other bonus is that it is easier to remove the posts if they are not cemented in the ground.

Parts List:

Zip Line Kit From Amazon. This one is made for kids but can hold up to 225 pounds. It comes with the trolley, cable, and most of the hardware that I needed. Cost $98.99 as of 7/18/2013.
Spring Swings Deluxe Fun Ride (Colors May Vary)

In the reviews for the Zip Line, people suggested attaching a disk seat swing to the trolley. This is a great Idea. The kids really like sitting on the disk seat while playing on the zip line. I bought the Little Tikes Disc Swing which is on Amazon for $19.99 as of 7/18/2013.

4x6" x 12' Pressure Treated Timber. Cost about $25.00 each from Home Depot.

Koch 3202093 2 Cable Block Fixed Flange, Zinc Plated Pulleys from Amazon. I bought 2 at first but needed an additional one to handle the high force on the starting side. Cost $6.99 each as of 7/18/2013.

Heavy Duty Chain - Campbell 0722327 System 3 Grade 30 Low Carbon Steel Proof Coil Chain on Reel, Zinc Plated, 3/8" Trade, 0.37" Diameter, 35' Length, 2650 lbs Load Capacity I used two 3 foot sections of this chain which I encased in concrete and rebar to form the ground anchors. Home Depot sells the chain by the foot for about $2 per foot.

Light Chain I used two 10' sections, Home Depot will cut to length for about $1.50 per foot.
Koch A01211 1/4 by 20-Feet Grade 30 Proof Coil Chain , Zinc Plated

Concrete - I used 4 bags of Sakrete 80lb Concrete Mix - $4 per bag at Home Depot 

Bonsal American Se (Wrb) 80Lbsakrete Concretemix 10280 Concrete Mix. You will need a container to mix the concrete and a measuring cup to measure the right amount of water as well as a shovel to mix it.

I used a post hole digger to dig the holes in the ground - It is difficult to go any deeper than 4' by hand.
Seymour Promotional Post Hole Digger with Wood Handle DG-60

 

1 Cubic Yard of 3/4" crushed stone from local stone yard. $30 delivered.

Six 1/4" Stainless Steel or Zinc Coated Anchor Shackles - about $2-4 each from Home Depot

Forney 61161 Screw Pin Anchor Shackles, 1/4-Inch

1/2" Rebar for the cement anchor, I cut it up into 2-3 foot sections and used 2 pieces per anchor for a total of about 12'. Cost about $15-$20 total depending on where you buy it. I had some pieces lying around from a previous project.
Steelworks Boltmaster #4-1/2X72 Rebar 11804 Rebar W/Smooth Rods

Since the cable can break at the loop, I reinforced the loops with thimbles for added security. The kit had one so I bought one for the other end of the cable.Koch 143121 Wire Rope Clips and Thimble for 1/8-Inch Cable, Zinc Plated. Cost $4 on Amazon as of 7/18/2013.

It is very hard to get the proper initial tension by hand. The final tension is set with the turnbuckle which comes in the kit. I used a ratcheting cargo strap for this. Cost $19.10 for a set of 4 from Amazon as of 7/18/2013. 

TEKTON 6223 15-Foot by 1-Inch Premium Ratchet Tie Downs, 4-Piece

Zip-line build with no trees

Disclaimer: Use your own judgement or consult a professional before installing your own zip line. This blog describes my thought process, plans, and implementation. I do not claim or guarantee that this information is correct or appropriate for your implementation. Zip lines may cause injury or even death. Install and use at your own risk.

I found a great item on Amazon that will provide entertainment for our kids in our backyard. It is a Zip Line designed for kids. It is meant to be strung between two trees, supports up to 225 pounds and gives a maximum ride of 70 feet.

My big problem with installing a Zip Line is that my backyard has no trees. I looked around and could not find instructions for setting up a zip line with posts. There are a number of challenges with using posts.

 First is the height difference. Based on the instructions that came with the kit, for the 60 foot run, the zip line needs about 30 inches of height difference. The starting height should be about 102 inches and the ending height would be about 72 inches.
I used a laser level (SKIL MT 8201-Self-Leveling Cross Line Laser) and a yard stick to measure the difference in elevation between the start and end of the zip line run in my backyard. Luckily, I had about 22 inches of elevation change. This means that I only need 8 additional inches of height differential for my set-up.

Second is the tension in the cable. The kit uses 1/8" steel cable with a tension of 300 pounds with no load and 600 pounds maximum tension. It would be very difficult to set posts in the ground that could withstand that amount of force without bending over and losing tension.
I thought of two solutions to the tension problem. 1. Brace the posts with struts or wire anchored to the ground. 2. Have the cable anchor to the ground on both ends and go over the posts with pulleys. I chose option 2 since it seemed easier to implement.

Here is a diagram of my plans:

As you can see, most of the tension is borne by the concrete anchors in the ground. The posts are subjected to a bending force that depends on the angle of the cable as it bends around the pulley. The further away from the post that the anchor is placed, the less bending force (and overall force) on the post.
This design can be further improved by angling the posts away from each other by 5 or 10 degrees.